The present invention provides a system for chemically conditioning an alkaline etching solution before etching wafers to inhibit the contamination of wafers during an etching process by metal contaminants in the solution.
The production of silicon wafers for integrated circuit substrates requires the use of complex processes involving many mechanical and chemical steps. Achieving both an extremely smooth and flat wafer surface and a high degree of purity in the ultimate product are two of the most important concerns in the process. A flat surface is necessary to ensure good pattern transfer from a photomask or reticle to a wafer. Modern lithographic techniques may require sub-micron images to be projected onto the surface of a wafer from a photomask or a reticle. To focus such a small image on the wafer surface, the distance between the photomask or reticle and the wafer surface must be carefully controlled. Typically, the photomask or reticle pattern will be transferred to the wafer surface by an optical aligner such as a step-and-repeat projection aligner. A step-and-repeat projection aligner focuses a reticle pattern on only one portion of a wafer at a time. After a portion is exposed, the image field is stepped across the surface of the wafer, and exposure is repeated at each step. Any variations in the shape, or flatness, of the wafer surface across the surface of the wafer may cause problems in this process. For instance, variations in the wafer surface shape may result in a poor transfer of a reticle pattern if the image is not refocused between exposure fields. On the other hand, if the image is refocused after each step, throughput may be decreased due to the time expenditure of refocusing. Because wafer flatness is so important to integrated circuit fabrication, each step in the wafer production process must be designed with the goal of increasing or preserving wafer flatness.
Several general steps are used to produce silicon wafers from a single crystal ingot of silicon. First, individual wafers are sliced from the end of the ingot. Next, the edges of the wafer are shaped to reduce the incidence of chipping during normal wafer handling. After edge shaping, a lapping step is used to bring the wafers within a specified thickness tolerance, and to reduce bow and warp. The wafer typically has 1 micron or less variation in surface shape across its diameter after lapping. Each of these processes may contaminate or damage the wafer surfaces, so a wet etching step is performed to etch away the contaminated and damaged layer. Generally an acid etch using hydrofluoric and nitric acids is employed. In this process, the nitric acid attacks the silicon to form SiO2, which is then dissolved by the hydrofluoric acid. A diluent such as water or acetic acid may be used to control the concentration of the rate-determining species to vary the etching rate. Acid etching is a predictable and well-characterized process, but offers poor control of wafer surface shape, and may cause surface shape variation on the order of 2 microns or greater. Thus, the flatness of the wafer may be reduced, possibly making the wafers unsuitable for some lithographic processes.
Alkaline etches are preferable to acid etches when more control over the wafer surface shape is desired. In an alkaline etching process, silicon wafers are immersed in a solution of a strong base such as sodium hydroxide. The silicon is oxidized by the hydroxide to form the water-soluble SiO32xe2x88x92 ion, which is then removed from the wafer surface by dissolution into the etching solution. The sodium hydroxide etching process offers superior shape control over acidic etching processes, typically causing a surface shape variation of 1 micron or less. However, the sodium hydroxide etching process presents problems with contamination not encountered in the acidic process.
Minimizing contamination is always a concern in the wafer production process. Single crystal ingots from which individual wafers are cut are extremely pure, with a concentration of impurity atoms typically on the order of 1010 atoms/cm2. Because the reliable performance of devices fabricated on silicon substrates is dependent upon the extremely high purity of the substrate, this purity level should be maintained throughout the wafer production process. Every processing step is a potential source of contamination. For instance, if contaminants are present in an etching solution, they may diffuse into the wafers during an etching process. Though many measures are taken to keep etching solutions as free of contaminants as possible, contaminants may enter the solution in several ways. One potential source of contaminants is the etching container itself. Etching containers, or baths, are generally constructed of stainless steel. Some of the solutions used in etching processes, such as the sodium hydroxide solution used for alkaline etches, can leach metals such as nickel from the steel walls of the baths. Furthermore, the acids or bases used to mix the etching solutions may contain some impurities. Any wafer immersed in these solutions may be contaminated by these impurities unless some mechanism is used to inhibit the contamination.
One way to inhibit the contamination of wafers in an etching process is to choose a process with a greater etch rate than the diffusion rate of the contaminants into the wafers. Acid etches have this advantage; the rate of etching of silicon in an HF/HNO3/HOAc solution is greater than the rate of nickel diffusion into silicon. Sodium hydroxide etching, however, occurs at a slower rate than the diffusion of nickel into silicon. Thus, wafers etched with sodium hydroxide are vulnerable to nickel contamination, and some method of inhibiting contamination must be used.
The contamination levels of wafers etched with a particular batch of etching solution tend to decrease as the batch is used for more etchings. For instance, high concentrations of nickel may be found in wafers that have been etched with a freshly-mixed solution of sodium hydroxide, while much lower concentrations of nickel may be found in wafers etched with well-used solutions. As a result, the earlier batches of wafers etched with a freshly-mixed etching solution may be unsuitable for use as substrates. This effect may lower system throughput and efficiency, as a portion of the total number of wafers processed with each batch of solution may not be useable due to contamination problems. Thus, it would be desirable to have a system for preventing the contamination of wafers with metal contaminants when using a freshly-mixed etching solution during an alkaline etching process.
One aspect of the invention provides a system for conditioning an alkaline etching solution for reducing contamination in a silicon wafer etching process. The system includes a conditioning tank for mixing a conditioned alkaline etching solution, the conditioned alkaline etching solution including a conditioning chemical; a conditioning chemical introduction system configured to add the conditioning chemical to the conditioning tank; and a buffer tank for storing the conditioned alkaline etching solution before using the conditioned alkaline etching solution in an etching process.
Another aspect of the present invention provides a system for conditioning an alkaline etching solution to reduce contamination in a silicon wafer etching process. The system comprises means for holding the alkaline etching solution while the alkaline etching solution is being conditioned, means for adding a conditioning chemical to the alkaline etching solution while the alkaline etching solution is in the holding means, and means for storing the alkaline etching solution after the conditioning chemical has been added to the alkaline etching solution.